1. Field of the Invention
The present invention relates to methods for the manufacture of finned heat transfer tubing, and, more particularly, to a method for the cold working of heat transfer tubing prior to the attachment thereto of a spiral fin, or fins, as one or several continuous lengths of metal strip are wound around the tubing.
2. Description of the Prior Art
The manufacture of heat transfer tubing through the winding of fins onto smooth tubing is known and practiced in a variety of ways. Though initially employed only in connection with non-ferrous tubing and fin profiles, because of the inferior ductility of steel strip, winding methods have been developed recently by which steel strip material can be wound onto tubing of a round or oval cross section. Such a method and device are disclosed in U.S. patent application Ser. No. 569,887, filed Apr. 21, 1975.
The present invention addresses itself to the specific problem which arises in connection with the winding of fins onto round tubing, when the winding tension on the last fin spiral is relaxed, either as the result of termination of the winding operation, or as the result of strip fracture. When this happens, the fin coil opens and uncoils a very small amount from its wound position, as a result of its residual bending elasticity. In the case of certain materials, especially steel strip, this residual elasticity may be just enough to eliminate the frictional engagement between the supporting edge of the fin coil and the smooth outer surface of the round tubing, thereby rendering the tubing unsuitable for its intended use.
In the past, this problem has been dealt with by attaching the metal strip to the tubing with spot welds at the beginning and end of each winding operation and prior to cutting of the metal strip. The same spot welding operation was then also necessary later, when a length of finished tubing was cut in two, for example.
However, no counter-measures against the accidental unwinding of the fin coil in the case of strip fracture during the winding operation have been available up to now. Obviously, such strip fractures can lead to very serious interruptions in the manufacturing process, when the fin coil on the tubing snaps open. The distance over which such an opening action may extend can amount to several yards of tubing. Such occurrences are not only costly in terms of time lost, they also can lead to sizable material losses.